The present invention relates to a metal foil composite sheet for making a hole for use in printed circuit board fields and a method of making a hole which method uses the above sheet and is capable of making a hole in high quality. More specifically, it relates to a lubricant sheet for making a hole which lubricant sheet serves to improve quality and productivity with regard to drilling works in printed circuit board fields and, when a hole having a diameter of 2 mm or less is made, which lubricant sheet prevents heat generation of a drill bit by a frictional heat, improves location accuracy and is capable of making a hole in high quality and at high efficiency, and a method of making a hole in a printed circuit board which method uses the above lubricant sheet.
U.S. Pat. Nos. 4,781,495 and 4,929,370 disclose methods of making a hole in a printed circuit board with a drill in which, when a hole for conduction between front and reverse surfaces is made in a printed circuit board with a drill, one surface or both surfaces of the printed circuit board is/are provided with a sheet obtained by impregnating paper or the like with a water-soluble lubricant, specifically which is a mixture of a synthetic wax which is a solid water-soluble lubricant and is formed of glycols such as diethylene glycol or dipropylene glycol and a fatty acid with a nonionic surfactant. However, these methods have defects. The defects are that the effect of preventing a heat generation of a drill is insufficient, that the impregnation of a porous sheet with the above mixture is poor and that the sheet is sticky.
As means for solving the above defects, JP-A-4-92494 publication and JP-A-6-344297 publication propose hole-making methods using a lubricant mixture sheet composed of polyethylene glycol or polyether ester and a water-soluble lubricant.
The above methods improve the quality of a drilled hole and provide an improvement in the stickiness. However, when the thickness of a layer of a lubricant is thick, a metal foil composite sheet composed of the lubricant and a metal foil warps largely in some cases, so that the above methods have a problem with workability. When a sheet is integrated with a metal foil, for maintaining bonding properties it is required to bond the sheet to the metal foil at a temperature where the sheet is sufficiently softened. As a result, the warpage of the resultant composite sheet tends to increase by the influence of a thermal shrinkage, which causes a problem with productivity or workability. Further, there is found a tendency that location accuracy of a hole decreases with decreasing the diameter of a drill bit to be used. Therefore, improvements are required for applying the above methods when a drill having an extreme small diameter is used.
It is an object of the present invention to provide a lubricant sheet for making a hole for a printed circuit board which lubricant sheet is excellent in the effect of preventing heat generation of a drill bit and which lubricant sheet is free from stickiness and a method of making a hole which uses the above sheet.
It is another object of the present invention to provide a lubricant sheet for making a hole for a printed circuit board which lubricant sheet is almost free from the occurrence of warpage, gives a hole excellent in quality and can improve workability and a method of making a hole which method uses the above sheet.
It is further another object of the present invention to provide a lubricant sheet for making a hole which gives a hole having excellent form and which can maintain a fine location accuracy of a hole even when a hole is made with a drill bit having a diameter of 0.3 mm or less and a method of making a hole which uses the above sheet.
According to the present invention 1, there are provided a lubricant sheet for making a hole used for a printed circuit board, comprising an organic substance layer having a thickness of 0.02 to 3.0 mm and formed of a mixture (a) or a mixture (b) and a metal foil having a thickness of 0.05 to 0.5 mm,
said organic substance layer being formed on one surface of said metal foil,
said mixture (a) being a mixture containing 20 to 90 parts by weight of polyether ester (I), 10 to 80 parts by weight of a solid water-soluble lubricant (II), the total amount of the polyether ester (I) and the solid water-soluble lubricant (II) being 100 parts by weight, and 2 to 10 parts by weight of polyethylene glycol (III) having a number average molecular weight of 200 to 600, and
said mixture (b) being a mixture containing 20 to 90 parts by weight of said polyether ester (I), 10 to 80 parts by weight of said solid water-soluble lubricant (II), the total amount of the polyether ester (I) and the solid water-soluble lubricant (II) being 100 parts by weight, and 2 to 20 parts by weight of a liquid water-soluble lubricant (IV),
and a method of making a hole which uses the above lubricant sheet.
According to the present invention 2, further, there are provided a lubricant sheet for making a hole used for a printed circuit board, comprising a layer formed of a water-soluble polymer and having a thickness of 0.02 to 3.0 mm and a metal foil having a thickness of 0.05 to 0.5 mm,
said water-soluble polymer layer being formed on one surface of said metal foil, and
the lubricant sheet having at least one characteristic selected from (a), (b) and (c),
(a) said water-soluble polymer layer is formed of a mixture of the water-soluble polymer and an organic filler (V),
(b) the surface of said metal foil which surface is to be bonded to said water-soluble polymer layer has a surface roughness of from 5 to 15 xcexcm, and
(c) said metal foil is an aluminum foil having an aluminum purity of at least 99.5%,
and a method of making a hole which uses the above lubricant sheet.
In the present invention 2, there is preferably used as a water-soluble polymer layer an organic substance layer composed of from 20 to 90 parts by weight of polyether ester (I) and from 10 to 80 parts by weight of a solid water-soluble lubricant (II).